The present invention relates generally to fiber optics communication apparatus, and more specifically to an improved fiber-optic switch using a Risley prism pair to steer a light beam emitted from any one of M input optical fibers to any one of N output optical fibers.
The switch provides low insertion loss over a wide spectral bandwidth, a high degree of robustness and stability, and moderate microprocessor-controlled switching speeds (tens of msec) for values of M and N as high as a few hundred. It also provides the ability to self-align individual fiber channels as necessary to compensate for mechanical or thermal drift. Applications include multi-channel sampling switches for polling-type monitoring instruments, network rerouting and bypassing switches, and wavelength-selective add/drop switches.
Typically, fiber-optic switches are implemented on a small scale (i.e., only one or two input and output fibers per switch) either opto-mechanically (by such means as physically moving bare fiber tips into alignment with each other for proximity coupling, butt coupling), or electro- or thermo-optically, (using interference in optical waveguides to switch between channels). The small-scale switches may then be combined in a switching xe2x80x9ctreexe2x80x9d to achieve larger values of M and N (in an Mxc3x97N array). Because of the extremely tight tolerances necessary for efficient fiber coupling, existing opto-mechanical switches tend to be expensive, unreliable, and not very robust. Optical waveguide switches are more robust, but have high insertion loss and poor stability. Moreover, the assembly of small-scale switches into a tree exacerbates the insertion losses, which are multiplicative in the number of elements.
Although there are several optical methods available for steering a beam of light, Risley prisms offer significant advantages in terms of sensitivity and stability. Risley prisms change the deflection angle of a light ray by rotation of the prisms about an optical axis. The prisms typically require a large rotation angle to change the deflection angle by a small amount; that is, the rotational motion of the Risley prisms is demagnified in its effect on the deflection angle. The demagnification is not constant, but varies with the magnitude of the deflection angle. For example, as shown below, a Risley pair with 1xc2x0 maximum deflection has a demagnification of fifty-seven or greater (magnification of 0.0175 or less); that is, a change in deflection angle of one arcsecond requires rotating the Risley prism about the optical axis through almost one arcminute or more. Because of the demagnification, and because of the insensitivity of the deflection angle to changes in the other degrees of freedom (tilt or displacement) of the prisms, Risley prisms provide extremely stable beam pointing.
Conventional optical methods for beam steering are much more sensitive to the mechanical stablity of the steering element because they do not provide the inherent demagnification of the Risley prisms. For example, in the usual method of beam steering with a tiltable mirror, the deflection angle of a light ray reflected from a mirror changes by twice the change in tilt angle of the mirror; that is, the mirror has a magnification of two (demagnification of one half). To achieve comparable stability in the direction of the steered beam, the thermal and mechanical stability of the mirror and its mounting structure must be more than one hundred times better than the stability of the Risley prism switch.
Another commonly used beam steering approach is to decenter a lens in the beam path. In this case, the magnification is one, so the stability requirements are similar to those of the tiltable mirror. This stability is often hard to achieve in an adjustable mount.
It is therefore an object of the present invention to utilize the positive characteristics of the Risley prism to provide an improved optical switching mechanism that is relatively inexpensive, is robust and exhibits good stability, and has relatively low insertion loss.
Briefly, a single channel Mxc3x97N optical switch in accordance with the present invention is provided which utilizes Risley prism techniques as means to steer light radiating from any one fiber in an input bundle into any selected fiber in an output bundle. More specifically, precision beam-steering optics is implemented using a Risley prism pair controlled by a small computer. In order to efficiently couple light between the two single-mode optical fibers a low-aberration lens approximately collimates the light from the source fiber before it is passed through the Risley prisms. The light beam passing through the prisms is approximately collimated in order to minimize the aberrations introduced by the prisms as they collectively deflect the beam through some precise angle. A second low-aberration objective lens is used to refocus the light at the core of a selected output fiber. The focusing lens transforms the angular deflection of the beam into a change in the position of the focused spot in the focal plane of the lens. In order to switch the light beam from one fiber in the output bundle to another, the Risley prisms are individually rotated about the optical axis to predetemined orientations so as to deflect the light from the input fiber at the exact angle necessary to direct the focused spot to the core of a selected output fiber.